fix: ensure bytes cannot go negative on requests sent to the server (#…

…300)

This can happen if the server sends a message that goes over the byte limit, which should never happen, but apparently does in edge cases. This ensures that the client doesn't fail if this happens.

fixes: #294

google/cloud/pubsublite/internal/wire/flow_control_batcher.py

@@ -20,6 +20,14 @@
 _MAX_INT64 = 0x7FFFFFFFFFFFFFFF
 
 
+def _clamp(val: int):
+    if val > _MAX_INT64:
+        return _MAX_INT64
+    if val < 0:
+        return 0
+    return val
+
+
 class _AggregateRequest:
     _request: FlowControlRequest.meta.pb
 
@@ -39,10 +47,13 @@ def __add__(self, other: FlowControlRequest):
         return self
 
     def to_optional(self) -> Optional[FlowControlRequest]:
-        if self._request.allowed_messages == 0 and self._request.allowed_bytes == 0:
+        allowed_messages = _clamp(self._request.allowed_messages)
+        allowed_bytes = _clamp(self._request.allowed_bytes)
+        if allowed_messages == 0 and allowed_bytes == 0:
             return None
         request = FlowControlRequest()
-        request._pb = self._request
+        request._pb.allowed_messages = allowed_messages
+        request._pb.allowed_bytes = allowed_bytes
         return request
 
 

tests/unit/pubsublite/internal/wire/flow_control_batcher_test.py

@@ -42,3 +42,17 @@ def test_add_remove():
     restart_2 = batcher.request_for_restart()
     assert restart_2.allowed_bytes == 5
     assert restart_2.allowed_messages == 1
+
+
+def test_negative_bytes_not_negative_request():
+    batcher = FlowControlBatcher()
+    batcher.add(FlowControlRequest(allowed_bytes=10, allowed_messages=3))
+    restart_1 = batcher.request_for_restart()
+    assert restart_1.allowed_bytes == 10
+    assert restart_1.allowed_messages == 3
+    batcher.on_messages(
+        [SequencedMessage(size_bytes=10000), SequencedMessage(size_bytes=3)]
+    )
+    restart_2 = batcher.request_for_restart()
+    assert restart_2.allowed_bytes == 0
+    assert restart_2.allowed_messages == 1